Download and decompress the images/ and inc/ folders at your MyBB installation’s htdocs/ directory (or you could simply decompress everything and then remove the install/ folder), which by default would be /opt/bitnami/apps/mybb/htdocs/.

To change your MyBB Board’s default language, enter the Administration Control Panel and navigate to “Configuration -> General Configuration”, then change the default language to the one you just installed:

To change your MyBB AdminCP’s language, navigate to “Configuration -> Control Panel Preferences (Global)” and change the language to the one you just installed: